1. Field of the Invention
This invention relates to spectrally sensitized photothermographic materials of the dry silver type and to their preparation.
2. Prior Art
Photothermographic materials of the dry silver type comprise a light-insensitive silver source material, usually a silver salt of a long chain fatty acid; e.g., silver behenate, as the heat sensitive component and a silver halide as the light sensitive component. The silver halide may be prepared in situ by conversion of a portion of silver soap by reaction with halide ions or it may be preformed and added during soap preparation.
Dry silver materials are generally sensitized by the addition of spectral sensitizing dyes prior to coating the silver halide/silver soap mixture on a support. Suitable dyes include the cyanine type sensitizers, well known in the spectral sensitization technology of conventional silver halide systems, and merocyanine type sensitizers as disclosed, for example, in U.S. Pat. Nos. 3,719,495 and 3,716,279 and U.K. Pat. No. 1,466,201. The dyes are generally added in solution to the silver halide/silver soap mixture. This is an inefficient method of sensitizing due to the fact that much of the sensitizing dye is adsorbed by the non-light sensitive silver soap, thus enabling only a proportion of the dye added to be adsorbed by the light-sensitive silver halide. Accordingly, large concentrations of dye must be used in order to ensure the adsorption to the silver halide of the required proportion of dye. The necessity for such large amounts of dye in the silver halide/silver soap mixture has limited the number of suitable dyes, mainly to merocyanines, since many known spectral sensitizing dyes will not heat bleach effectively at high concentrations thereby causing background staining of the material.
In the case of dry silver materials having a preformed silver halide component, an alternative method of spectral sensitization comprises treating the silver halide with the sensitizing dye prior to mixing with the silver soap. An example of this technique is disclosed in U.S. Pat. No. 4,105,451 in which light sensitive silver halide is spectrally sensitized before addition to the heat sensitive silver salt of a heterocyclic thione. The spectral sensitization of the preformed silver halide suffers from the disadvantage that the dye becomes easily desorbed from the silver halide surface during the silver soap preparation and the homogenization stages, where large variations in temperature, pressure, pH and the addition of solvents are encountered. This results in the sensitizing effect in the dry silver material becoming diminished or in some cases completely destroyed.
The technique of preparing silver halide emulsions in the presence of a sensitizing dye is known for use in conventional silver halide systems. U.S. Pat. No. 2,735,766 discloses blending a spectral sensitizing dye of the merocyanine type with either the silver salt or the halide salt prior to mixing these two salts to form silver halide. U.S. Pat. No. 4,225,666 discloses introducing a sensitizing dye into the reaction vessel after the nucleation of the silver halide grains has occurred and before the silver halide preparation is completed. Neither of these references disclose the use of such spectrally sensitized silver halide emulsions in photothermographic materials.
Research Disclosure No. 19,227 generally discloses techniques for preparing silver halide emulsions. This reference states that the grain growth or crystal habit modifiers, metal dopants or species capable of creating internal sensitivity centers and/or spectral sensitizing dyes, desensitizing dyes or desensitizing organic compounds (electron acceptors) can be present in the reaction vessel or the reactant(s) stream(s) prior to initiation of nucleation, during nucleation, after nucleation is complete, throughout the entire precipitation or just prior to completion of precipitation. This reference further states that silver halide emulsions prepared in the presence of grain growth or crystal habit modifiers, metal dopants or species capable of creating dyes, desensitizing dyes or desensitizing organic compounds (electron acceptors) can be used in color, color transfer, black-and-white negative, graphic arts, photothermographic, cine positive, cine negative, documentary and recording, and radiographic materials, etc. However, there is no experimental data to substantiate any of the statements made nor any disclosure of a specific photothermographic material in this reference.
Heretofore, the range of sensitizing dyes used in photothermographic materials has been limited for the reasons discussed above. In particular, J-banding cyanine dyes which have been used extensively in conventional silver halide systems have not been used effectively in photothermographic materials with the exception of anhydro-9-ethyl-3,3'-di(3-sulphopropyl)-4,5;4',5'-dibenzothiacarbocyanine hydroxide sodium salt which is disclosed in U.S. Pat. No. 3,871,887 in a photothermographic material in the presence of lithium iodide which is said to promote J-aggregation.
It is an object of the present invention to provide a process for preparing spectrally sensitized photothermographic materials in which a wide range of spectral sensitizing dyes may be used, including J-banding dyes, and to provide photothermographic materials incorporating a wide range of spectral sensitizing dyes including J-banding dyes.